vector1234
Member
We are in the process of replacing a 500 kva 480/277 transformer with a 2000 kva transformer. Originally, a 1500 kva transformer was going to be used. The ownner installed 4 sets of 4 conductor 750 KCMIL CU THWN-2 (MTW) conductors, only hooked up two sets for the 500 and left two curled up in the vault below the transformer. 4 sets would have worked for the 1500 kva transformer but not the 2000 kva unit, it needs 6 sets of phase conductors. The transformer essentially serves motor loads with very small if any neutral current, There are six 5" PVC underground conduits to the transformer (duct bank to vault under transformer from main switchgear). The feeder length is about 35 ft. (switchgear to transformer) Technically the existng 4 750 KCMIL neutral conductors are more than adequate, however, it looks like the code will require us to install two additional neutral conductors or two more sets of 4 750 kcmil conductors. This is a retrofit and this is where the code often breaks down. We did not design the original installation, if so we would have down sized the neutral conductor (3 750 kcmil phase, 1 ~500 kcmil neutral), in duct bank phase conductors can run separate from neutral conductors, but I do not think we can make this case. I would like to install just 2 additional sets of 3 750 kcmil in two conduits (ABC) and leave 4 conduits with (ABCN) but I do not think the code allows this. Basically we end up with 6 750 kcmil CU connected to a neutral carrying almost no current. Not an economical and/or practical arrangement. Any ideas? Thanks